"Welcome to the human race. Nobody controls his own life, Ender. The best you can do is choose to fill the roles given you by good people, by people who love you."
Valentine explains to Ender her view of human life. She is telling him that he must look past the fact that people have used throughout his life. She says that everyone uses everyone else, and it is not possible to simply follow your own path. However, we can choose to follow a path that someone we love has set out before us. What the ones we love want us to do may be good for us, and it may even be what we want to do. It is human freedom that is at stake, and it cannot be claimed in a vacuum. But it is possible to feel free within a path that is not entirely of one's own making, and that is what Valentine tries to tell Ender. He may not completely agree with her, because Ender wishes to be totally free, but in the end he knows that we all must serve someone. He chooses to serve the buggers because he wants to—the path they set out for him is the one that corresponds with his freedom.